The invention relates to an output stage for an integrated E.C.L. circuit, said output stage comprising:
a first and a second transistor which are interconnected by way of their emitters and which form a differential circuit which is connected to a first power supply line and which is connected in series with a first current source to which the emitters of both transistors are connected and which is connected to a second power supply line, the collector of the first transistor being connected to the first power supply line via a first resistor,
a third transistor whose emitter forms the output of the circuit and whose base is connected to said first resistor and the collector of the first transistor,
a fourth transistor whose base and collector are connected parallel to the base and the collector, respectively, of the first transistor and whose emitter circuit comprises a second current source which is connected to the second power supply line.
Herein, the letters "E.C.L." indicate a technique which is known from the field of logic integrated circuits; these letters correspond to the initial letters of the words "Emitter Coupled Logic".
The output levels 0 (low) and 1 (high) should be accurate and as stable as possible voltage levels in a logic circuit. The voltage levels on the output, however, inevitably vary as the operating temperatures of the elements of the circuits vary, notably the temperatures of the p-n junctions of the transistors of this circuit. Therefore, logic circuits are customarily "temperature-compensated" in order to restrict such output voltage variations.
A circuit designed for this purpose and intended for the output stage of an E.C.L. circuit is shown in FIG. 4 of an article published in the magazine "IEEE Journal of Solid Stage Circuits" (Vol. SC 14, No. 5, October 1979). This circuit is based on the use of a current source (corresponding to the "second current source" as indicated above) which supplies an auxiliary current I.sub.A which flows via a resistor (the "first resistor" above) and which causes a voltage drop across this resistor whose variation has an opposite polarity with respect to the variation of the base-emitter voltage V.sub.BE of the output transistor ("third transistor"). Because, when taken with respect to the general reference potential (the potential of the power supply line to which said transistor is connected), the output level equals the sum of the voltage drop across this resistor and V.sub.BE of the output transistor and because these two parameters vary in opposite directions as a function of the temperature, a given compensation can be achieved. Actually, beyond a given temperature (approximately 60.degree.) of the p-n junctions of the transistors overcompensation occurs, the increase of the voltage drop in the transistor then being larger than the amount whereby the V.sub.BE of the output transistor decreases; thus, the voltage level of the high and the low output levels decreases as the temperature increases.